A damaged spillway that diverts overflow from a California reservoir has authorities preparing to use a forested emergency spill route if Lake Oroville keeps rising. A salmon hatchery below the dam is at risk, officials say.

Rising levels of Northern California's Lake Oroville, a reservoir contained by the Oroville Dam, the largest dam in the US, led to severe erosion of a paved spillway that helps divert overflow water into the Feather River behind the dam.

On Tuesday, a chunk of the spillway collapsed, sending mud and debris into the river, as officials with the California Department of Water Resources (DWR) temporarily ceased the overflow route to inspect the problem. Yet flows had to continue as the reservoir continued to rise. The section of collapsed spillway has grown to more than 200 feet wide and 30 feet deep, according to the San Francisco Chronicle. If the reservoir continues to balloon, authorities say they will have to use an emergency spillway, which is essentially a nearby wooded hillside.

As of now DWR is not expecting to use emergency spillway, but clearing debris as a contingency.

"DWR shut off flows temporarily to investigate but has resumed using the gated spillway channel in order to try to avoid having the lake rise to elevation 901 feet above sea level, the point at which water naturally flows over a concrete weir and down a wooded hillside," the DWR said in a statement Friday.

The DWR says the public is not currently at risk, as the US Army Corps of Engineers has said the main dam is sound.

"None of this emergency we have now is affecting the integrity of the dam itself," said Bill Croyle, deputy director of the DWR. "I think that’s important ... there’s no risk to the public."

Nevertheless, authorities have told residents near the Feather River to be prepared for a potential emergency evacuation, Reuters reported.

More precipitation on Thursday pushed reservoir operators to increase flow to the eroded spillway from 40,000 cubic feet per second (cfs) to 65,000 cfs, DWR said.

"The higher flows further eroded the gated spillway channel, but not to a point that would compromise the integrity of the dam upstream," the department said. "DWR and many federal and state partners are monitoring the spillway erosion carefully and are prepared to ramp down flows if necessary."

Crews are currently clearing more than 50 acres of brush and debris along the hillside in order to limit what flows into the Feather River in case the emergency spillway is used, officials said, according to the Chronicle.

The DWR said the emergency spillway has not been used since the 770-foot Oroville Dam, a major source of water throughout the state, was constructed 48 years ago. Lake Oroville came within about a foot of the emergency track in January 1997, the agency said.

Meanwhile, the Feather River Fish Hatchery, the source of about seven million Chinook salmon and steelhead per season, has taken in severe water turbidity levels as a result of the overflow and damaged spillway. On Thursday, turbidity, or cloudiness, levels were about 40 times above normal, according to Harry Morse, spokesman for the California Department of Fish and Wildlife.

"This is just uncharted territory," Morse said, according to the Chronicle.

For fear of mass fish kills amid the debris- and mud-filled water flowing into the hatchery, state Fish and Wildlife crews have rushed to transport about four million baby salmon to a downstream hatchery that is safe from rapid inflows. An additional four million fish will have to remain at the hatchery, Morse said.